1. Field of the Invention
The present invention relates generally to a spark ignition timing control system for an automotive internal combustion engine for adjusting spark advance magnitude for optimal engine performance. More specifically, the invention relates to a spark ignition timing control system specifically adapted for controlling spark ignition timing in engine operation combustioning a mixture fuel, in which more than one type fuels, such as gasoline and alcohol, having mutually different combustion characteristics.
2. Background of the Invention
It is recent trend in the automotive technology to seek for higher anti-polution performance for protecting or recover pure atmosphere. Particularly, in the automotive vehicle exhaust gas, significant reduction of polutant, such as No.sub.x, CO and so forth, is becoming more and more strong order for this purpose. As can be appreciated, from the past, various attempts have been made for purification of engine exhaust gas. Such attempts gain significant level of improvement in terms of anti-polution. However, in view of increasing of automotive vehicles, further purification of exhaust gas becomes strictly important.
From the past, it has been known in the automotive technologies that alcohol fuel may create much lesser polutant, such as NO.sub.x, CO and so forth, in comparison with petrol or qasoline. Since alcohol fuel can be industrially or artificially produced, hi bride fuel engines have already been developed and put into the market in certain countries. Such hi-bride fuel engine utilizes a blended fuel of gasoline and alcohol. In such blended fuel, combustion characteristics of the fuel varies significantly depending upon gasoline/alcohol mixture ratio due to relatively low combustibility of alcohol. Therefore, in case of such blended fuel, fuel delivery amount in terms of an induction air flow rate, spark ignition timing and so forth need to be controlled adapting the blended fuel property.
As is well known, spark ignition timing controls are performed principally for seeking maximum spark advance angle for best torque in view of optimization of engine output performance. Such MBT control is combined with knock control in which spark advance angle is adjusted for maintaining the engine in light-knock condition. For harmonizing both of the MBT control and the knock control, Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-14479 proposes spark ignition timing control for controlling spark advance angle with respect to respective of individual engine cylinders. Namely, in the proposed system, knocking resistance at each individual engine cylinder is experimentally checked. MBT control is then performed in view of a cylinder pressure data in an engine cylinder having the highest knocking resistance until unacceptable engine knocking is detected. When unacceptable engine knocking is detected at the aforementioned specific engine cylinder having the highest knocking resistance, then the engine cylinder having second highest knocking resistant is selected to use the cylinder pressure data thereof for MBT control. Such proposal may be effective for enabling advancing of spark advance in most of the engine cylinders at MBT point.
The aforementioned prior proposal may be only applicable for the spark ignition timing control in terms of homogeneous fuel, i.e. gasoline, engine. In case of blended fuel, the proposed procedure of spark advance control cannot be directly applied due to substantial variation of MBT points depending upon gasoline/alcohol blending ratio. Substantial variation of MBT points may not significantly affect for the spark ignition timing control performance as far as steady or stable engine operating condition is concerned since the spark advance angle can be gradually adjusted toward the MBT point by feedback control. However, on the other hand, in the engine transition state, such as accelerating and decelerating state of the engine, offset of spark advance angle from MBT will cause substantial degradation of spark ignition timing control performance. Namely, when the spark advance angle is set at far retard angle from MBT point, the engine output can be lowered than that can be obtained at MBT point, to cause lowering of engine acceleration performance during engine acceleration state. On the other hand, excess magnitude of spark advance in terms of MBT point will leads unacceptable engine knocking.
In view of such difficulty, Japanese Patent First Publication (Tokkai) Showa 61-85578 proposes setting of variable basic spark advance in terms of property of fuel to be combustioned. For this, a fuel sensor for detecting mixture ratio or concentration of one of individual fuels in the blended fuel to select one of a plurality of settings. In case of gasoline/alcohol blended fuel, since the fuel property changed substantially according to the concentration of alcohol versus gasoline, substantially large capacity of memory becomes necessary for pre-setting various basic spark advance with respect to all possible variation range of alcohol mixture ratio.